The present invention relates to recording apparatus, and more particularly to electrostatic recording or electrophotographic copying apparatus which comprises an image carrier such as a toner image-carrying photoreceptor drum, etc., and an intermediate image transfer material, e.g., in the belt form, onto which is transferred the toner image on and from the image carrier. The transferred image on the intermediate transfer material is further transferred onto a sheet material, or a subsequent image transfer material, under a heated condition, and the image transfer section where the image transfer is made onto the foregoing image transfer material is comprised of rotating means (e.g., a roller) for the image transfer and another rotating means (e.g., a roller) for bringing the transfer material into pressure contact with the rotating means for the image transfer.
In conventional electrostatic or electrophotographic recording apparatus, the image formation is generally carried out in accordance with the process that an electrostatic image is formed on an image carrier such as a photoreceptor drum, the electrostatic image is then developed by a developer comprised of a toner or a mixture of a toner and a carrier, and the resulting toner image is electrostatically transferred onto an image transfer sheet, and further the transferred image is fixed.
However, in the case where the toner image is transferred electrostatically onto a transfer sheet (a recording material) by a transfer electrode such as, e.g., a corona discharger, the image transfer process is accompanied by a turbulence of the electrostatic charge, deteriorating the resulting toner image. Further, if a recently recommended conductive electromagnetic toner or the like is used as the toner, a further turbulence of the electrostatic charge is produced, thus causing a substantially untransferable condition.
For improving the above described disadvantage in the electrostatic image transfer, attempts have been made to use a pressure roller to press a toner image onto a transfer sheet, but the method has the drawback that the transfer efficiency is unsatisfactory with the resulting toner about half transferred in density.
Hereupon, there has been proposed a method wherein the above toner image is pressed to be transferred onto an intermediate transfer material in the belt form comprising rubber as the transfer layer, and the transferred toner image is then pressed to be transferred and fixed onto a transfer sheet under a heated condition by use of a heating roller, as described in Japanese Patent Examined Publication Nos. 41679/1971 and 22763/1973, Japanese Patent Publication Open to Public Inspection (hereinafter referred to as Japanese Patent O.P.I. Publication) No. 78559/1974 and U.S. Pat. No. 3,993,825, and the like. In such a method, a toner image is pressed to be transferred onto the surface layer of an intermediate transfer material, the surface layer being comprised of a rubber-type material such as, e.g., silicone rubber, fluororubber, or the like, having, on one hand, mold releasability and, on the other hand, adherence to toner particles when subjected to pressure. The toner image on the intermediate transfer layer is brought into contact with and heated by a heating means such as a heating roller to become fused and concurrently pressed to be transferred and fixed onto a fed-in transfer sheet. Namely, because the toner image fused by heating is readily transferred according to the above mold releasability and fixed onto a transfer sheet, the method brings about no deterioration of the toner image resolution due to the transfer process, and further realizes a highly efficient image transfer operation.
In the recording apparatus of the prior art, the above transfer-fixing section is comprised of a heating roller and a pressure roller for pressing against the heating roller, and an intermediate transfer material in the belt form and a moving sheet material (transfer paper) are passed being pressed by and between these rollers, whereby the toner image on the intermediate transfer belt is transferred and fixed onto the transfer paper (recording paper). In this instance, if a heat source is arranged also inside the pressure roller and both rollers are heated together, both intermediate transfer belt and recording paper can be maintained at a sufficient temperature for transferring and fixing the same, so that the process can become an excellent measure for improving the transferability as well as fixability.
FIG. 1 is a schematic illustration of conventional recording apparatus having an intermediate transfer belt. In the region along the periphery of image carrier 1 such as a rotary drum-type photoreceptor drum are provided known latent image forming mechanism 2, developing mechanism 3 and cleaning mechanism 4 in the described order in the rotating direction of the drum. In transfer region A between developing mechanism 3 and cleaning mechanism 4, intermediate transfer material 5 in the endless belt form is brought into pressure contact with the above-mentioned image carrier 1 by pressure (transfer) roller 6. Intermediate transfer belt 5, as shown in FIG. 2, consists of flexible film base plate 20 such as of a polyimide and an adhesive material, e.g., silicone rubber layer 21 coated thereon. The above intermediate transfer belt 5 is suspended over pressure roller 6, heating roller 7 and tension roller 8, and moves in transfer region A in the same direction and at the same speed as those of toner image carrier 1 toward intermediate transfer material heating region B of heating roller 7. Cleaning roller 12 is provided facing opposite to tension roller 8, and further neutralizer member 13 is provided for neutralizing the electrostatic charge of belt 5. At the parting point of intermediate transfer belt 5 from heating roller 7 or in transfer-fixing region C in the proximity of the parting point is provided pressure roller 9 so as to press on heating roller 7. Transfer sheet (recording paper) heating plate 10 is provided along transfer sheet heating region D immediate before transfer-fixing region C in transfer sheet moving path P set up so as to pass transfer-fixing region C.
In the above apparatus, the toner image formed on the image carrier is finally transferred and fixed onto the image transfer sheet in the following manner:
Toner image T on toner image carrier 1 is first formed by developing by means of developing mechanism 3 of the latent image formed by latent image forming mechanism 2. That is, in the case of utilizing electrophotography, image carrier 1 composed of such a photoreceptor as of the selenium, organic compound, zinc oxide or cadmium sulfide binder type, or the like, is used to be formed thereon with an electrostatic latent image by charging the entire peripheral surface of image carrier 1 by means of latent image forming mechanism 2, and then subjecting the charged area to an imagewise exposure. In the case of utilizing the electrostatic recording process, image carrier 1 composed of a dielectric consisting of a conductive substrate and dielectric surface layer coated thereon is used to be formed thereon with an electrostatic latent image converted from an imagewise signal by means of a multi-stylus electrode or of an ion control electrode. Alternatively, image carrier 1 composed of a magnetic material may be used to be formed thereon with a magnetic latent image converted by means of a magnetizing head from an imagewise signal.
The thus formed latent image, if it is an electrostatic image, is visualized by developing mechanism 3 with a toner of colored particles charged in the polarity opposite to that of the charge forming a latent image. If the toner used herein is a one-component conductive magnetic toner, the development is made by the charge induced by the toner. Where the development is made by use of the one-component conductive magnetic toner in the magnetic brush developing process, a layer of the toner can be formed as a single-particle layer or as much thin a layer as close to it, so that this method is particularly favorable in respect that it can produce rapidly the final image excellent in the quality as well as in the durability thereof, and enables to carry out a satisfactory image transfer without selecting a transfer material. If the latent image is a magnetic latent image, a magnetic toner should be used to develop the latent image. The use of the magnetic toner produces no blur at the time of the image transfer, thus producing a clear image.
The thus formed toner image T on image carrier 1 is transferred in transfer region A by the pressure of pressure roller 6 onto intermediate transfer belt 5. The adhesive layer, particularly silicone rubber layer, of intermediate transfer belt 5, due to its appropriate surface adhesiveness and rubber elasticity enough to gain the toner at a low temperature, can attract sufficiently to capture the toner to the intermediate transfer belt side, overcoming the toner retainability of image carrier 1 in transfer region A. The surface energy of the silicone rubber layer is sufficiently small as compared to ordinary image transfer materials, so that when, in transfer-fixing region C which will be described hereinafter, the transfer sheet, under the condition that the toner is heated to become fluid, is pressed on the intermediate transfer belt, the toner becomes strongly adhering to the transfer sheet, whereby the image can be almost completely transferred and fixed. Heating roller 7 is a hollow metallic cylinder such as of aluminum having thereinside a heater 7a such as, e.g., an infrared lamp. The surface temperature of this metallic roller is controlled so as to fall under such an appropriate temperature range that in the region between the contact starting point E with intermediate transfer belt 5 and transfer-fixing region C; that is, in the intermediate transfer belt heating region B, the intermediate transfer belt 5 with toner image T thereon is heated, and the toner image is sufficiently transferred and fixed in region C. Pressure roller 9 in transfer-fixing region C is provided as an auxiliary heating roller for improving the transferability and fixability, and is a heat-resistant elastic roller provided therearound with a surface layer such as of a silicone rubber and also provided thereinside with a heater lamp 9A. Transfer material heating plate 10 shown in the example of FIG. 1 is in such a form as adaptable to be in contact with the periphery of pressure roller 9, and, when the above transfer sheet passes between the surface of pressure roller 9 and the transfer material heating plate 10, the heating plate heats the transfer sheet to such an extent that the toner image on intermediate transfer belt 5 is sufficiently transferred and fixed onto the transfer sheet in transfer-fixing region C. The coefficient of friction of the surface of pressure roller 9 is made larger than that of the surface of transfer material heating plate 10, so that with the movement of the surface of pressure roller 9 due to the revolution thereof, the transfer sheet rubs the surface of transfer material heating plate 10 to be heated and transported to transfer-fixing region C. The thus heated transfer sheet, in transfer-fixing region C, passes being pressed by and between the intermediate transfer belt 5 in advance heated together with the toner image by heating roller 7 and the pressure roller 9, whereby the toner of the toner image fluidized by the heating is pressed to be transferred and fixed onto the transfer sheet.
The transfer sheet that has passed transfer-fixing region C is generally transported along intermediate transfer belt 5 and separated from intermediate transfer belt 5 by tension roller 8. If tension roller 8 is small in diameter, the separation of the transfer sheet from intermediate transfer belt 5 may be more easily made. Further by swaying intermediate transfer belt 5, possible deviation of the belt can be prevented.
Intermediate transfer belt 5 that has passed transfer-fixing region C is partially naturally cooled, and again subjected to the subsequent image transfer at transfer region A, and further at transfer-fixing region C the transfer-fixing process is repeated.
In FIG. 1, 11 is a cleaning roller which, when pressure roller 9 is stained with the toner, removes the toner from the roller.
In the recording apparatus of FIG. 1, a recording paper as the transfer sheet is heated to a sufficient temperature for image transfer and fixing when passing between the transfer roller 7 and the pressure roller 9 (i.e., the transfer-fixing section), and the period of time required for the passing of the recording paper is relatively short. In order to obtain a sufficient temperature for the transfer and fixing during this short period, it is necessary to raise the temperature of the transfer roller and pressure roller to a considerably higher degree than the toner softening starting temperature. The temperature of the transfer roller and pressure roller being a little higher than the toner softening starting temperature is not considered sufficient for the transfer and fixing of the recording paper, so that no desired transfer-fixing process may be carried out. On the other hand, however, if the temperature of the transfer roller and pressure roller is increased to be excessively higher than the toner softening starting temperature, the power consumption becomes unfavorably increased.
Upon this, as shown in FIG. 1, a heating plate 10 as means for in advance heating the recording paper was devised to be provided in the proximity of pressure roller 9 so that a sufficient temperature for the transfer and fixing of the recording paper can be obtained before the recording paper is put in between transfer roller 7 and pressure roller 9. The means, heating plate 10, allows the temperature of the transfer roller and pressure roller to be lower than the abovementioned temperature, thus enabling to save the power consumption.
However, it has now been found that because heating plate 10 is composed of a rigid material having no flexibility, when a recording sheet passes between the heating plate 10 and pressure roller 9, the recording sheet cannot be brought into close contact with heating plate 10 (further with pressure roller 9), so that the temperature of the recording sheet become uneven locally. That is, even if the recording sheet passes being nipped between the heating plate 10 and pressure roller 9, because heating plate 10 is rigid, it is impossible to make even the interfacial contact with the recording sheet, and besides, this is furthered by the dispersion of the disposition accuracy of heating plate 10 and of the relative dimensional accuracy between the heating plate 10 and pressure roller 9. Besides, changes in the width of the path for the recording sheet can occur due to the roatation shake of pressure roller 9, the changes also hindering the uniformity of the interfacial contact or smooth advance of the recording sheet. However, in such the in-advance heating, for example, the recording sheet is to be transported with being in contact with the pressure roller that presses the recording sheet against the heating plate over an extensive area, so that there is produced a large friction between the sheet and the pressure roller, thereby tending to cause the sheet to jam.